Unusual organization of scent glands in Trogulus tricarinatus (Opiliones, Trogulidae): evidence for a non-defensive role

Abstract

The morphology of the scent glands of Trogulus tricarinatus (Linnaeus 1767) (Trogulidae), a small, soil-dwelling opilionid species, was investigated by means of serial histological semi thin-sections. The glands constitute paired prosomal glandular sacs that open to the body surface via one pore (ozopore) on either side of the body, dorsally adjacent to coxae I. Consistent with the generally recognized organization of scent glands in Opiliones, an anterior non-secretory region of the reservoir could be distinguished from a posterior secretory area, the latter characterized by a thick vacuolated epithelium. However, there are several unusual scent gland features in T. tricarinatus. First, the ozopores are hidden, with each being surrounded by a kind of external secretion atrium formed by a dorso-lateral integumental fold (dorsal limitation), coxa I (ventral limitation), and a wall of projecting cuticular papillae (outer lateral limitation). A horizontal slit ("secondary opening") between the top of this wall and the dorsal integumental fold is externally visible. Secondly, no fluid, but solid spherical structures that may represent condensed secretion are found in the reservoirs. Thus, the secretion must pass through the external atrium before reaching the outside, perhaps as a gas produced by slow sublimation of solid secretion boli. Scent gland organization in T. tricarinatus, especially the findings of an external atrium around the ozopores, is not consistent with use in chemical defence, as is generally assumed for scent glands of Opiliones, but indicates a possibly nondefensive role.

Nitrogen-Containing Compounds in the Scent Gland Secretions of European Cladonychiid Harvestmen (Opiliones, Laniatores, Travunioidea)

“…Such amounts correspond to a volume of 15 nl of nicotine (which is a liquid with a density of 1.01 g/ml at room temperature), and hence to two hypothetical spherical “droplets” of about 130 μm in diameter. Moreover, droplets of that size would be consistent with the expected dimension and filling capacity of glandular reservoirs in Holoscotolemon , at least if glandular dimensions in other opilionids of comparable size are considered (e.g., Juberthie, 1976; Schaider and Raspotnig, 2009). In general, the whole body extraction-method is widely used and commonly accepted for the assessment of exocrine products in small arthropods that are too tiny to sample or dab their secretions directly from the gland orifices.…”

Functional anatomy of scent glands in Paranemastoma quadripunctatum (Opiliones, Dyspnoi, Nemastomatidae)

“…Also some Cyphophthalmi are known to emit secretion only on the tubercle of the body side where they are irritated, though emission on both tubercles has also been observed (Juberthie, 1961a). While these observations clearly imply a defensive function of scent glands in Laniatores and Cyphophthalmi, many additional hypotheses on the biological role of aberrant dyspnoid scent glands meanwhile exist—these range from defensive functions over territorial marking to sex pheromone production (e.g., Holmberg, 1986; Juberthie et al, 1991; Schaider and Raspotnig, 2009). At least for P. quadripunctatum , the primary function still seems to be defense against predators along with microbial protection, and this view is well supported by both, the functional anatomy of glands and the typical defensive chemistry of the secretions.…”

“…They generally consist of large hollow sacs provided with glandular tissue and they are known to produce different classes of rather volatile, in some species highly scented compounds (e.g., Ekpa et al, 1984, 1985; Shear et al, 2010; Pomini et al, 2010; Raspotnig et al, 2005, 2010; Föttinger et al, 2010). Beyond these common features, scent gland organization and chemistry greatly differs among the phyletic lineages of Opiliones, rendering the gland systems well suited to study evolutionary traits of opilionid groups on different taxonomic levels: Typical defensive glands obviously only occur in cyphophthalmids and laniatoreans (Gnaspini and Hara, 2007), whereas scent glands in the Eupnoi and even more in the Dyspnoi are frequently modified, rather inconspicuous or even hidden (e.g., Schaider and Raspotnig, 2009). Unlike cyphophthalmids and laniatoreans, especially dyspnoids appear to be reluctant to emit secretion, and in some groups—such as in Troguloidea—secretion discharge cannot be observed at all, not even after heavy mechanical stimulation (e.g., Pabst, 1953).…”

“…Unlike cyphophthalmids and laniatoreans, especially dyspnoids appear to be reluctant to emit secretion, and in some groups—such as in Troguloidea—secretion discharge cannot be observed at all, not even after heavy mechanical stimulation (e.g., Pabst, 1953). Moreover, some dyspnoids such as Ischyropsalididae and some Trogulidae were found to store secretion in solid form in glandular reservoirs, which is a very unusual, unique feature for defensive glands (e.g., Lopez et al, 1980; Juberthie et al, 1991; Schaider and Raspotnig, 2009). The mechanism of emission of secretion in these species has been explained by volatilization of compounds due to sublimation processes, speculatively resulting in the generation of a defensive, chemical shield around the body (Juberthie et al, 1991; Gnaspini and Hara, 2007).…”

Interspecific, ontogenetic, and sexual variation in ozopore morphology among cosmetid harvestmen (Arachnida, Opiliones, Laniatores)

“…With approximately 6,600 species, the order Opiliones represents the third largest group of arachnids and is currently divided into the suborders Cyphophthalmi, Dyspnoi, Eupnoi, and Laniatores (Pinto‐da‐Rocha & Giribet, ). One of the most striking synapomorphies of harvestmen is the occurrence of paired exocrine scent glands along the margins of the carapace, dorsal to coxae II that produce chemical secretions that may function as secondary defenses (reviewed by Gnaspini & Hara, ), alarm cues (Machado, Bonato, & Oliveira, ), or in social communication (Schaider & Raspotnig, ). The chemical composition of scent gland secretions vary phylogenetically and may be composed of quinones, phenols, or ketones (Raspoting, Schaider, Föttinger, & Schönhofer, ; Raspotnig et al, ).…”